Calling it "an impossible dilemma," Doctors Without Borders explained in detail their decision in response to a New York Times story on the case. It would have been the first time the experimental drug was tried in humans.

The explanation came the same day that another top doctor from Sierra Leone died of the disease, further fueling a debate about how to apportion a limited supply of untested drugs and vaccines and whether they are even effective.

Ebola has killed more than 1,000 people and sickened nearly 2,000 in the current West African outbreak that has also hit Guinea, Liberia and Nigeria. Many of the dead are health workers, who are often working with inadequate supplies and protection.

At the time the experimental treatment was being considered for Dr. Sheik Humarr Khan, his immune system was already starting to produce antibodies suggesting he might recover, Doctors Without Borders said in the statement. Khan was also due to be transferred to a European hospital that would be more capable of handling problems that might arise, the organization said.

The experimental drug, ZMapp, is designed to boost the immune system to help it fight the virus. Since Khan's body was already producing an immune response, the doctors may have feared that any boost would kick it into overdrive.